On Wed, 24 Jul 2024 18:14:56 +0200
Miguel Ojeda <ojeda@kernel.org> wrote:
> The Rust compiler added support for `-Zfunction-return=thunk-extern` [1]
> in 1.76.0 [2], i.e. the equivalent of `-mfunction-return=thunk-extern`.
> Thus add support for `MITIGATION_RETHUNK`.
>
> Without this, `objtool` would warn if enabled for Rust and already warns
> under IBT builds, e.g.:
>
> samples/rust/rust_print.o: warning: objtool:
> _R...init+0xa5c: 'naked' return found in RETHUNK build
>
> Link: https://github.com/rust-lang/rust/issues/116853 [1]
> Link: https://github.com/rust-lang/rust/pull/116892 [2]
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
> ---
> arch/x86/Makefile | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index e8214bff1aeb..a1883a30a5d8 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -24,11 +24,15 @@ RETPOLINE_CFLAGS += $(call cc-option,-mindirect-branch-cs-prefix)
>
> ifdef CONFIG_MITIGATION_RETHUNK
> RETHUNK_CFLAGS := -mfunction-return=thunk-extern
> +RETHUNK_RUSTFLAGS := -Zfunction-return=thunk-extern
> RETPOLINE_CFLAGS += $(RETHUNK_CFLAGS)
> +RETPOLINE_RUSTFLAGS += $(RETHUNK_RUSTFLAGS)
> endif
>
> export RETHUNK_CFLAGS
> +export RETHUNK_RUSTFLAGS
> export RETPOLINE_CFLAGS
> +export RETPOLINE_RUSTFLAGS
> export RETPOLINE_VDSO_CFLAGS
>
> # For gcc stack alignment is specified with -mpreferred-stack-boundary,
> @@ -218,6 +222,7 @@ KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
> # Avoid indirect branches in kernel to deal with Spectre
> ifdef CONFIG_MITIGATION_RETPOLINE
> KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)
> + KBUILD_RUSTFLAGS += $(RETPOLINE_RUSTFLAGS)
> # Additionally, avoid generating expensive indirect jumps which
> # are subject to retpolines for small number of switch cases.
> # LLVM turns off jump table generation by default when under